The present invention relates to methods and apparatus for processing signals to remove redundant information thereby making the signals more suitable for transfer through a limited-bandwidth medium. The present invention specifically relates to methods and apparatus useful in video compression systems.
Many signal processing techniques useful in video compression systems are known. For example, digital encoding is often employed in processing television signals which are to be transferred over transmission channels since digital data streams are more immune to noise degradation.
In order to digitally encode a television signal, a signficant number of bits, 4 or more, may be required to provide for an acceptable range of gray scale for each of the hundreds of thousands of separate picture elements (pixels) which form an image. Consequently, data rates for unprocessed digitalized television signals typically require a bandwidth greater than 40 megabits per second. If the communications link is an earth satellite, an unprocessed video signal typically occupies nearly the entire bandwidth of the satellite, with very few channels, if any, left over for other uses. A T1 communication channel is typical and has only a 1.5 megabit per second bandwidth. A practical yet effective way to reduce the bandwidth of digitalized television signals is needed so that fewer channels are required for transmission over a communications path and so that the quality of transmitted signals is maintained even when reduced bandwidth transmission is employed.
U.S. Pat. No. 4,302,775, assigned to the same assignee as the present invention, describes a scene adaptive coding technique which eliminates redundant information and thereby reduces the bandwidth.
The patent describes a single-pass digital video compression system which implements a two-dimensional cosine transform with intraframe block-to-block comparisons of transform coefficients without need for preliminary statistical matching or preprocessing.
Each frame of the video image is divided into a predetermined matrix of spatial subframes or blocks. The system performs a spatial domain to transform domain transformation of the picture elements of each block to provide transform coefficients for each block. The system adaptively normalizes the transform coefficients so that the system generates data at a rate determined adaptively as a function of the fullness of a transmitter buffer. The transform coefficient data thus produced is encoded in accordance with amplitude Huffman codes and zero-coefficient runlength Huffman codes which are stored asynchronously in the transmitter buffer. The encoded data is output from the buffer at a synchronous rate for transmission through a limited-bandwidth medium. The system determines the buffer fullness and adaptively controls the rate at which data is generated so that the buffer is never completely emptied and never completely filled.
In the system receiver, the transmitted data is stored in a receiver buffer at the synchronous data rate of the limited-bandwidth medium. The data is then output from the receiver buffer asynchronously and is decoded in accordance with an inverse of the encoding in the transmitter. The decoded data is inversely normalized and inversely transformed to provide a representation of the original video image.
The U.S. Pat. No. 4,302,775 patents reduces redundancy by employing intraframe coding techniques utilizing intraframe comparisons of cosine transform coefficients. While the patent provides significant improvement over other techniques, there is a need for even greater compression.
In addition to intraframe coding techniques, interframe coding techniques have been used to reduce the rate required for video transmission as described, for example, in the above-identified application. Typically, each video frame is held in memory at both the transmitter and the receiver and only frame-to-frame changes are transmitted over the communication link. In contrast to intraframe coding schemes in which the quality of coded images is dependent upon the amount of detail in each single image frame, the quality of the coded image in interframe coding is dependent upon the differences from frame to frame. Frame-to-frame differences are often referred to as "motion".
Interframe coding techniques are broadly classified into two categories, namely, spatial domain coding and transform domain coding. In real-time interframe spatial-domain coding systems, spatial domain data can be threshold processed to obtain and store frame difference signals in a transmitter buffer. The threshold value can be adaptively determined as a function of the transmitter buffer fullness. In order to eliminate the image breakdown, both spatial and temporal subsampling has been proposed.
The above-identified U.S. patent application entitled "A Combined Intraframe and Interframe Transform Coding System" employs intraframe and interframe variable prediction transform coding. Images are represented by sequential frames of two-dimensional arrays of digital signals. The digital signals are transformed to form transform coefficients for each frame. Predicted transform coefficients are formed using sets of variable prediction factors. The predicted transform coefficients for each frame are compared with corresponding actual transform coefficients for the frame to form transform coefficient difference signals. The difference signals are processed to control their range of values. The processed difference signals are statistically coded such that the more frequently occurring values are represented by shorter code lengths and the less frequently occurring values are represented by longer code lengths. The coded signals are stored in a buffer memory for transmission. The coded signals in the buffer memory are transmitted, over a limited-bandwidth medium, to the receiver along with processing information. The processing information includes codes identifying the set of variable prediction factors utilized in the transmitter. The same set of variable prediction factors is utilized in the receiver to reconstruct predicted transform coefficients which in turn are used to reconstruct representations of the original images in the transmitter.
The extension of the Scene Adaptive Coding of U.S. Pat. No. 4,302,775 from intraframe coding to interframe coding has proven very significant in terms of improving image quality and reducing bandwidth. These improvements, however, have created a need for improved coding systems for reducing redundancy and there continues to be a need for improved signal processing methods and apparatus for data compression systems.